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Dart / Generics and Types

Extension types

Learn extension types in Dart through a focused explanation, a dedicated topic-specific example, an engineering boundary, and a mastery check.

12 min read • Intermediate

extension types is an atomic Dart topic inside Generics and Types. The example uses the named construct directly so you can see its syntax, data flow, and observable behavior.

Read the code by locating the exact feature named extension types, then trace the value before and after that feature runs. This lesson is intentionally isolated from neighboring topics: changing identifiers alone would not reproduce the same behavior because the construct itself is different.

Topic-specific example

dart-0056-ee55910700.dart
dart Try It Live 
extension type UserId(String value){}void main(){var id=UserId('u1');print(id.value);}
Edge case / engineering note

For extension types, test the boundary that makes the feature meaningful: invalid input, an empty value, a missing branch, lifecycle/cleanup, or a type/runtime constraint. Do not infer correctness only from the happy path.

Content source & attribution
Original Syntaxis Content · Original Syntaxis Content

Lesson quizzes

Extension types Mastery Check →
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